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Role of Genes in Shaping Intelligence Is Lifelong, Study Says

Psychologists have long assumed that the relative importance of genes in shaping a person's intelligence declines over the years, because the other determinant of intelligence, experience, increases as a person ages.

But a new study of 240 pairs of twins, all older than 80, demonstrates that genes are just as important for cognitive function in old people as they are in middle-age adults. The investigation, which is being published today in the journal Science, found that the genetic contribution to an old person's intellect is about 50 percent, the balance being attributable to education, stress exposure, occupation, socioeconomic status, geography, nutrition, disease and all the other environmental factors that shape life.

A statement by the National Institutes of Health, which sponsored the study, predicted that further study of the role of genes in intelligence in older people ''could lead to beneficial interventions that might slow or reverse cognitive decline,'' including the ravages of Alzheimer's disease.

The two-year study was carried out by a team of Swedish, British and American scientists under the leadership of Dr. Gerald E. McClearn, director of the Center for Developmental and Health Genetics at Pennsylvania State University, under a grant from the National Institute on Aging.

Dr. Irving I. Gottesman of the University of Virginia in Charlottesville, who has investigated possible genetic causes of schizophrenia and other mental diseases, said in a comment published in Science that Dr. McClearn's study was a landmark demonstration that ''the genetic contribution to cognitive ability is remarkably constant throughout life.''

The statistical analysis that led to that conclusion was possible because several nations -- Australia, Denmark, Finland, Norway and Sweden among them -- maintain national registries of nearly all their twins.

Dr. McClearn and his collaborators at the University College of Health Services in Jonkoping, Sweden, the Karolinska Institute in Stockholm, the Institute of Psychiatry in London and Penn State chose their subjects from the Swedish Twin Registry. Of the 240 pairs, 110 were of identical twins and 130 were fraternal same-sex pairs, whose genes were only as similar to each other as those of ordinary siblings. All the twins were born before World War I, and their average age was 83.

Each twin in every pair was individually tested by a different nurse and was subjected to a demanding 90-minute battery of examinations aimed at measuring not only general intelligence, but also specific cognitive abilities. Those included tests of how long it took to complete various cognitive tasks, synonym matching, manipulation of colored cubes to match patterns presented on cards, identifying like and unlike figures, memorizing strings of numbers and recognizing pictures that had been shown earlier.

The analysis of the results showed large differences between the degree of cognitive matching in the two types of twins, identical or fraternal. In general, as expected, the identical twins were much closer to each other intellectually than were the fraternal twins. Previous large-scale testing of twins, including a major investigation a decade ago by Dr. Thomas J. Bouchard Jr. and his colleagues at the University of Minnesota, yielded similar results.

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''But what is so new and surprising about the latest study,'' said Dr. Jared B. Jobe, chief of the Adult Psychological Development Branch of the National Institute on Aging, ''is that it shows the genetic influence on cognitive ability to remain very strong, even in old age.''

Dr. McClearn said in an interview that another study by his group, the Swedish Adoption Twin Study on Aging, had established the importance of genes in old-age cognitive ability in another way. That investigation focuses on identical twins who were reared apart and therefore share relatively little common experience. Like similar studies on younger twins, the study has revealed the importance of genes in cognitive ability throughout life, he said.

Rather than declining with age, the role of heredity in cognitive ability seems to increase.

In 1993, a group of scientists led by Dr. Matthew McGue of the University of Minnesota published the result of a study of twins that suggested a steady rise in the lifetime role of heredity in cognitive function. They found that the genetic factor in general cognitive ability is about 20 percent in infancy, 40 percent in childhood, 50 percent in adolescence and 60 percent in adulthood. Dr. McClearn said his findings on the contribution of heredity in old age are consistent with the 1993 study.

The supposition that genes play an important role in the development of intelligence has aroused controversy since the 19th-century studies by Sir Francis Galton, a cousin of Charles Darwin who discerned a genetic influence as responsible for the intellectual similarity between identical twins.

Dr. McClearn and his colleagues said they believed that it was important to identify specific sites in genes that contribute to cognitive ability, even though there might be up to 10,000 such sites, each playing only a tiny part. But he does not advocate ''tinkering'' with the genes themselves. By developing chemical means to activate genes that exert desirable effects or deactivate genes that hasten cognitive decay, scientists may significantly improve the lives of old people, he said.

One such gene, called ApoE, has already come under suspicion as a factor in Alzheimer's disease.

The widely held belief that genes play at least some role in intelligence has been hotly contested in recent years, particularly by some critics who contend that human intelligence is molded almost exclusively by environmental factors.

''This field in general is somewhat controversial,'' Dr. Jobe of the National Institute on Aging said. ''But the new twin study focuses on gerontological issues and has nothing to do with longstanding debates over intelligence and heredity.''

Acknowledging that some social scientists believe that heredity plays virtually no role in the development of intelligence, Dr. McClearn said: ''They should look at the data. If you think about it, cognitive function depends on neurochemical processes in the brain, which are influenced by enzymes, which are made by genes. It would be dumbfounding for intellectual functioning to be without genetic influence.''